Wireless communication has grown to encompass a huge variety of information transactions between electronic machines. These include cellular communications between hand-held units and base stations, wireless communications between peer devices or master-servant components, and even between components on a same device.
For reliable interoperability, wireless communications have been organized into known formats, generally referred to by the associated protocols, so that multiple parties can communicate effectively using compatible communication devices and methods. This encompasses communications between devices employing same communication protocols but made by different manufacturers in different parts of the world. In some respects, these protocols determine the allowable or preferred techniques for delivering and interpreting data communicated between a plurality of communication devices. In other respects, the protocols govern the way in which information is packaged for transmission over conducting or optical lines or over the air (OTA) in a wireless communication environment.
Present and forthcoming systems provide relatively compact enclosures for performing the above design and testing. The enclosures are preferably relatively mobile and small in size compared to typical existing over-the-air (OTA) testing facilities, which are usually room-sized or laboratory-sized and not mobile. The enclosures are also preferably provide isolation from RF, microwave and other electromagnetic interference so that the testing conducted within the enclosures is substantially performed without such interference.
In some aspects, the forthcoming enclosure systems are geometrically and operationally adaptable for a variety of applications and uses, while maintaining a high degree of electromagnetic isolation. To this end, filtering of ambient electromagnetic interference (e.g., from laboratory electronics or surrounding interfering radios) is desirable.
Various structures commonly known as “filters” are used for suppressing or attenuating, to a desired specification, electromagnetic waves impinging on and propagating through the filter, depending on the signal's or wave's constituent frequencies. The number and scope of fields of communication, entertainment, and industrial equipment and systems requiring electronic filters is essentially indefinable. Therefore, it will be understood that the example applications for the filter described herein are not limiting; the fields are presented to assist the person of ordinary skill better understand the present filter, and to make and use a filter in accordance with described herein, for either an application similar to the example application, or any other of a wide range of applications.
As described in the prior art, including that cited herein, textbooks, technical journals, and other publications embody a large knowledge base of filters, including their types, structures, guidelines for selection, methods of design, construction, and testing. Within this large existing knowledge base, it is also well known that problems exist in designing and constructing a “low pass” filter, i.e., a filter that maintains high speed data integrity in the passband while at the same time effectively rejecting common wireless frequencies (e.g. in the microwave range). Furthermore, filtering Ethernet interfaces with Power over Ethernet (PoE) according to the IEEE 802.3af standard, is especially challenging since the filter must combine high frequency passband together with high current tolerance, which is challenging for traditional LC based filters. High power LC components are used to support the power present in the PoE interface and can have parasitic responses that limit their effectiveness when it comes to rejecting high frequencies (e.g., microwave wireless frequencies).
There are known methods and structures directed to filtering unwanted noise having frequencies above about 400 MHz (e.g., USB signal passband and up to, e.g., 6 GHz Wi-Fi band). One example is a miniature thin film filter as reported by Vion et al., J. Appl. Phys. 77, 2519 (1995). Another example is a distributed thin film microwave filter reported by Jin et al., Appl. Phys. Lett. 70, 2186 (1997). Still another example is the Philips Thermocoax filter, as discussed in A. Zorin, Rev. Sci. Instrum. 66, 4296 (1995). In most cases these filters were first used to reduce noise in single electron tunneling experiments. Perhaps the simplest and easiest to fabricate “microwave” filter is the bulky metal powder filter. The metal powder filter was first discussed in more detail by Martinis et al., Phys. Rev. B 35, 4682 (1987) and subsequently developed and discussed in detail by others. See K. Bladh et al., Rev. Sci. Instrum. 74, 1323 (2003), and A. Fukushima et al., IEEE Trans. Instrum. Meas. 45, 289 (1997).
The metal powder filters known in the relevant art have a central conductor that is surrounded by metal powder or a metal powder/epoxy mixture. The filter attenuates an incoming electrical signal via eddy current dissipation in the metal powder. The known art teaches, however, that the central conductor is shaped into the form of a spiral to increase the attenuation. This does indeed increase the attenuation but, as observed by the present inventors, these spiral conductor metal powder filters cannot be designed to have a characteristic impedance near 50Ω at high frequencies. It has been identified that the physical design of known metal powder low pass filters creates what is technically a short at high frequencies, not 50Ω. In many high frequency applications, however, it is useful to employ a matched 50Ω impedance measurement setup. If low pass filters are used they are also 50Ω in this scenario on each line of a differential transmission pair with the total impedance being 100Ω.
Further work in this space is disclosed in U.S. Pat. Nos. 7,456,702 and 7,791,430, and Patent Pub. No. US 2008/0284545, which are collectively and generally limited to coaxial designs of low pass metal powder filters and do not lend themselves for flexible and modern applications of differential 100Ω feed-through filtering systems.
Other shortcomings observed in testing environments are the prevalence of ferrite-core chokes. They come in many shapes, sizes and names. Beads, chokes, dongles, toroids, inductors, bumps, clamps, blocks, cores, rings and are all passive low-pass filters. The geometry and electromagnetic properties of coiled wire over the ferrite bead result in a high resistive impedance mostly for high-frequency signals, attenuating high frequency electromagnetic interference (EMI)/radio frequency (RF) electronic noise. When frequencies escalate, using this “choke” method often results in the energy reflected back up the cable.